Research Institute for Sustainable Humanosphere, Kyoto University
Generation processes of whistler-mode emissions in the magnetosphere
We first review the basic process of nonlinear wave growth of coherent whistler-mode waves with rising and falling-tone frequencies propagating parallel to the background magnetic field. The nonlinear wave growth takes place in a relatively short wave packet with a chirping frequency. Depending on the phase space density of resonant electrons trapped in a wave potential, the direction of the chirping changes. The frequency variation is due to formation of a resonant current parallel to the wave magnetic field. The frequency variation induces formation of a resonant current parallel to the wave electric field which causes nonlinear wave growth, resulting in formation of a new wave packet at a slightly different frequency. Through the formation of new wave packets repeated sequentially in time, a chorus element with a rising or falling-tone frequency is gradually formed along with its subpacket structure. We present a series of particle simulations of whistler-mode wave-particle interaction in a parabolic magnetic field with different frequencies of triggering waves and different plasma frequencies. Under a given plasma condition, we find that only a specific frequency range of the triggering wave can generate rising or falling-tone emissions. The generation regions of rising-tone emissions shift upstream from the equator, while those of falling-tone emissions shift downstream from the equator. For comparison, we also present simulations with a uniform magnetic field, where we find relatively weak rising and falling-tone emissions. We discuss roles of the magnetic field inhomogeneity and the frequency chirping in the nonlinear wave growth process generating whistler-mode chorus and hiss emissions in the magnetosphere.